Neural transmitters are chemicals in the brain that are used to send messages from one brain cell to another. Neurotransmitters bind to special receptor proteins in the membranes of nerve cells, like a lock in a key, triggering a chemical reaction within the cell. Dopamine is an example of a central nervous system (CNS) neurotransmitter.
Dopamine is a catecholamine belonging to a class of biogenic amine neurotransmitters, along with norepinephrine, serotonin, and histamine. The catecholomines (particularly dopamine and serotonin) are involved in the control of movement; mood; attention; and possibly, certain endocrine, cardiovascular, and stress responses. Imbalances in neurotransmitter production have been implicated in a variety of mental and physical disorders, such as Parkinson's disease (PD). It is thus desirable to diagnose and monitor such imbalances and to monitor the effectiveness of drugs and substances that affect brain chemistry.
New and powerful imaging methods that enable one to assess the living brain in vivo and thereby monitor brain chemistry and the effectiveness of drugs and substances that affect brain chemistry have been developed. Methods such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) involve administering to a patient a radioactive tracer substance comprising a ligand that binds to the presynaptic or postsynaptic neuroreceptors in the patient's brain. Emissions (primarily gamma rays are emitted from the positrons or photons from the radioactive tracer) are measured. These emissions are indicative of the number and degree of occupancy of blocking of the neuroreceptors. The number of neuroreceptors and the degree of occupancy or blocking is calculated utilizing a mathematical model, and compared with an intra-person or inter-person control to determine the degree of drug response. Further treatment of the patient with drugs is based on the comparisons made. For these methods to be useful, however, a ligand that has a high specificity and affinity for the desired receptor is required.
It is believed that certain radioactive ligands may be selective for dopamine transporters and are thus potentially useful in evaluating changes in dopamine function in vivo and in vitro, especially for patients with Parkinson's disease (PD), which is characterized by a selective loss of dopamine neurons in the basal ganglia and substantia nigra. Recently, a large number of dopamine transporter imaging agents based on cocaine or its closely related congeners, tropane derivatives, have been reported. (Carroll, F. I., et al, Med. Res. Rev. 1995, 15, 419-444; Carroll, F. I., et al., J. Med. Chem. 1994, 37, 2865-2873; Carroll, F. I., et al., J. Med. Chem. 1995, 38, 379-388). The regional brain distribution of cocaine is largely concentrated in the basal ganglia, where the dopamine neurons are located. [.sup.11 C]-N-methyl labeled cocaine (Yu, D.-W., et al., J. Med. Chem. 1992, 35, 2178-2183; Fowler, J. S., et al., Synapse 1992, 12, 220-227) is a very useful PET (positron emission computed tomography) ligand for studying the pharmacology and drug effects of cocaine itself; however, additional modifications on the cocaine molecule have led to development of positron emission tomography (PET) imaging agent, CFT (WIN35,428) (Clarke, R. L. et al. J. Med. Chem. 1973, 16, 1260-1267; Clarke, R. L. et al. J. Med. Chem. 1978, 21, 1235-1242; Frost, J. J., et al., Ann. Neurol. 1993, 34, 423-431; Wong, D. F., et al., Synapse 1993, 15, 130-142), and single photon emission computed tomography (SPECT) imaging agents .beta.-CIT (Innis, R. B., et al., Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11965-11969; Seibyl, J. P., et al., J. Nucl. Med. 1996, 37, 222-228; Kuikka, J. T., et al., Eur. J. Nucl. Med. 1995, 22, 682-686; Neumeyer, J. L., et al., J. Med. Chem. 1994, 37, 1558-1561.), IPT (Goodman, M. M. et al. J. Med. Chem. 1994, 37, 1535-1542; Mozley, P. D., et al., J. Nucl. Med. 1996, 37, 151-159), and other related derivatives that display much higher binding affinity and selectivity to dopamine reuptake sites. Both of the agents for PET and SPECT imaging displayed excellent specific uptake in the striatum (basal ganglia) area and are more suitable than GBR12,935 in imaging dopamine reuptake sites (dopamine transporters). (Kilbourn, M. R., Life Sci. 1988, 42, 1347-1353). The dopamine reuptake site ligands are useful in evaluating changes in dopamine reuptake sites in vivo and in vitro, especially for patients with PD. Recent publications describing the use of [.sup.11 C]-CFT (WIN35,428) (Frost, J. J., et al., Ann. Neurol. 1993, 34, 423-431) and [.sup.123 I]-.beta.-CIT (Innis, R. B., et al., Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11965-11969; Innis, R. B., Eur. J. Nucl. Med. 1994, 21, 1-5) suggest a strong correlation between the decrease in localization of dopamine transporters in the anterior putamen area and PD symptoms.
Currently, PET and SPECT imaging studies of dopamine transporters are under investigation. Recent publications using [.sup.11 C]-CFT and [.sup.123 I]-.beta.-CIT suggest a strong correlation between the decrease in localization in the anterior putamen area and PD symptoms. See Innis, R. B. et al. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 11965-11969; Innis, R. B. Eur. J. Nucl. Med. 1994, 21, 1-5; Frost, J. J. et al. Ann. Neurol. 1993, 34, 423-431, the disclosures of which are herein incorporated by reference in their entirety.
Central nervous system (CNS) receptor function has also been successfully evaluated in vivo using C.sup.11 (T.sub.1/2 =20 minutes, .beta.+) or F.sup.18 (T.sub.1/2 =120 minutes, .beta.+) labeled agents for positron emission tomography (PET) imaging and .sup.123 I (T.sub.1/2 =13 hours, 159 KeV) labeled agents for single photon emission computed tomography (SPECT) imaging. See Eckelman, W. C. Nucl. Med. Biol. 1992, 18, iii-v; Fowler, J. S. et al. Ann. Rep. Med. Chem. 1989, 24, 277-286; Fowler, J. S. et al. Ann. Rep. Med. Chem. 1990, 25, 261-268, the disclosures of which are herein incorporated by reference in their entirety.
A ligand that is being widely investigated as an agent for diagnosing and treating PD patients is [.sup.123 I]-CIT. However, one of the drawbacks of [.sup.123 I]-.beta.-CIT is the length of time (&gt;18 hours) required for reaching optimal uptake ratio in the target area (semi-equilibrium state) (the basal ganglia (BG)) versus the nontarget area (the frontal cortex (CTX)). Because of the need for agents with faster equilibrium times, new ligands such as [.sup.123 I]-IPT (N-(3-iodopropen-2-yl)-2.beta.-carbomethoxy-3.beta.-(4-chlorophenyl)tropan e) and [.sup.123 I]-.beta.-CIT-FP are under investigation, both of which reach equilibrium in less than one hour. See Kung, M.-P. et al. Synapse 1995, 20, 316-324; Malison, R. T. et al. J. Nucl. Med. 1995, in Press; Mozley, P. D. et al. J. Med. Chem. 1994, 37, 1558-1561, the disclosures of which are herein incorporated by reference in their entirety.
Despite the success in developing such new techniques using PET and SPECT for imaging CNS receptors, their use in routine procedures is hampered by the cost ([.sup.123 I] costs about $30/mCi) and the limited supply of the three isotopes mentioned above, .sup.123 I, .sup.11 C, and .sup.18 F, all of which are produced by cyclotron.
A radionuclide that is widely used in diagnostic nuclear medicine is technetium [.sup.99m Tc] (T.sub.1/2 =6 hours, 140 KeV). It is well established that when Tc-99m pertechnetate (TcO.sub.4 --), the most commonly available starting material, is reduced in the presence of a reducing agent, such as stannous chloride, and a "soft" chelating ligand, including N.sub.2 S.sub.2 and NS.sub.3, a [Tc.sup.v O].sup.3+ N.sub.2 S.sub.2 or [Tc.sup.v O].sup.3+ NS.sub.3 center core is formed. Technetium [.sup.99m Tc] is readily produced by a [.sup.99m Tc]/Mo-99 generator, and its medium gamma-ray energy emission (140 KeV) is suitable for gamma camera detection with a far less cost ($1/mCi). In the past ten years, significant progress has been made in defining technetium chemistry using the chemical level of [.sup.99 Tc] (T.sub.1/2 =2.1.times.10.sup.5 yr), and non-radioactive rhenium as a surrogate substitute, that will potentially benefit millions of patients who receive [.sup.99m Tc] agents for routine nuclear medicine diagnosis. Over 85% of the routine nuclear medicine procedures currently performed use radiopharmaceutical methodologies based on [.sup.99m Tc]. In addition, comparable rhenium complexes labeled with [.sup.186 Re] (T.sub.1/2 =90 hours) or [.sup.188 Re] (T.sub.1/2 =17 hours) may also be potentially useful for in vivo imaging of dopamine transporters.
The potential of developing [.sup.99m Tc] labeled agents for CNS receptor imaging is well recognized. Several recent reports demonstrate that it is possible to incorporate [TcvO].sup.3 +N.sub.2 S.sub.2 (bisaminoethanethiol, BAT) into potential receptor selective imaging agents for muscarinic receptors, vesamicol sites, and steroid hormone receptors. See Del Rosario, R. B. et al. Nucl. Med. Biol. 1994, 21, 197-203; Chi, D. Y. et al. J. Med. Chem. 1994, 37, 928-935; DiZio, J. P. et al. Bioconj. Chem. 1991, 2, 353-366; DiZio, J. P. et al. J. Nucl. Med. 1992, 33, 558-569; O'Neil, J. P. et al. Inorg. Chem. 1994, 33, 319-323; O'Neil, J. P. et al. Bioconj. Chem. 1994, 5, 182-193; Jurisson, S. et al. Chem. Rev. 1993, 93, 1137-1156; Jurisson, S. S. et al. Nucl. Med. Biol. 1995, 22, 269-281; Jurisson, S. et al. Chem. Rev. 1993, 93, 1137-1156; Steigman, J. et al. National Academy Press: Washington, D.C. 1992; Lever, S. Z. et al. Nucl. Med. Biol. 1994, 21, 157-164, Chi, D. Y. et al. Am. Chem. Soc. 1993, 115, 7045-7046, the disclosures of which are herein incorporated by reference in their entirety. However, these [.sup.99m Tc] imaging agents have demonstrated limited success in in vivo studies, this is believed to be attributable to the low initial brain uptake and poor selective binding to the receptor after attaching the molecules with [.sup.99m Tc].
Recently, a series of neutral and lipophilic conjugated complexes, containing N-alkylthiolatotropane, aminobisethylthiolato and a [.sup.99m Tc]TcO.sup.3+ center core, were prepared and evaluated as CNS dopamine transporter imaging agents in rats (Meegalla, S. K., et al., J. Am. Chem. Soc. 1995, 117, 11037-11038). One of the compounds, [.sup.99m Tc] technetium, [methyl 3-(4-chlorophenyl)-8-(2-mercaptoethyl)-8-azabicyclo [3.2.1]octane-2-carboxylato-S][[2,2'-(methylimino) bis[ethanethiolato]](2-)-N,S,S']oxo, displayed low initial uptake in rat brain (0.1% at 2 minutes post intravenous injection), but the striatal/cerebellar (ST/CB) ratio reached 3.50 at 60 minutes after an intravenous injection. The Rhenium complexes were also discussed. These neutral [.sup.99m Tc] labeled three plus one complexes designed for brain imaging, as well as mixed-ligand, aminothiol plus aminothiol complexes (two plus two complexes) designed as [.sup.99m Tc] steroid analogs are quite stable in vivo and in vitro. However, since these agents are not designed to cross the blood-brain barrier, they are inferior for imaging CNS receptors, such as dopamine and serotonin.
Technepine compounds containing technetium and rhenium have been investigated as potential dopamine transporter agents. Madras, B. K., et al., Synapse 1996, 22, 239-246. These compounds showed poor biodistribution, which is believed to be a result of the amide moiety present on the N.sub.2 S.sub.2 ligand.
Despite its attractive physical properties, technetium is a difficult element for designing suitable SPECT ligands. Technetium is a transition metal and requires a complexing agent to stabilize it at different valence states. Steigman, 1992, supra. Valance states can vary from plus 7 (as pertechnetate) to zero (0), depending on the reaction conditions and chelating agents used during preparation. After complexation, the molecules invariably become big and bulky, which is the limiting factor in designing a molecule targeted to a specific biological process(es). Additional requirements for Tc-99m labeled complexes as CNS receptor imaging agents are: i) small (molecular weight &lt;750), with good lipophilicity (partition coefficient .about.50-1,000); ii) high binding affinity (Kd &lt;10 nM) and high selectivity; iii) minimum brain uptake in rats should be 0.5% dose/organ at 2 minutes post intravenous injection.
The [.sup.99m Tc] brain imaging agents that have been developed thus far have been aimed at measuring perfusion or its changes due to a particular disease state, and not as diagnostics directed to evaluating neuronal functions, such as the biochemistry of dopamine or serotonin receptors. See Mastrostamatis, S. G. et al., J. Med. Chem. 1994, 37, 3212-3218; Spies, H. et al., Technetium and Rhenium in Chemistry and Nuclear Medicine, 1995, 4, 243-246; S. G. Editoriali, Ed.: Padova, Italy 1995, 4, 243-246; Spies, H. et al. Angew. Chem. Int. Ed. Engl. 1994, 33, 1354-1356; Chi, D. Y. et al. J. Amer. Chem. Soc. 1993, 115, 7045-7046; Chi, D. Y. et al. J. Med. Chem. 1994, 37, 928-935, the disclosures of which are herein incorporated by reference in their entirety.
Thus, there remains a need for new imaging agents, such as CNS receptor-based imaging agents, for evaluating neuronal functions that do not present the problems associated with prior agents, as discussed above. These agents should have good selectivity, affinity, and specific activity for the target. Several additional factors are also of importance such as radiochemistry (preparation time for short-lived labeled agents), suitable modeling for kinetics of receptor uptake and retention, and metabolism. The imaging agents should also be economical and readily available.
The present invention addresses these, as well as other needs, by providing novel tropane-based technetium- or rhenium- labeled imaging agents useful, inter alia, for imaging the CNS, in particular dopamine and serotonin receptors, to diagnose CNS abnormalities. It is also expected that the novel compounds of the invention possess pharmacological activity. It is believed that the compounds of the invention are the first imaging agents of their kind displaying specific regional uptake directly proportional to dopamine neuronal distribution in the brain.